Only prompt for a model when agent manifest includes a model resource

[therealjohn]

The init flow always asks for a model but should only do that if the manifest defines one as a resource.

[hund030]

Azd already conditionally skips model configuration.

Verified:

# no-model.yaml
template:
  kind: hosted
  name: no-model-agent
  protocols:
    - protocol: responses
      version: "1.0.0"

> azd ai agent init -m .\no-model.yaml

No model prompt and the generated azure.yaml looks like:

# yaml-language-server: $schema=https://raw.githubusercontent.com/Azure/azure-dev/main/schemas/v1.0/azure.yaml.json

name: prompt-model
services:
    ai-project:
        host: azure.ai.project
    no-model-agent:
        project: .
        host: azure.ai.agent
        language: docker
        docker:
            remoteBuild: true
        uses:
            - ai-project
        container:
            resources:
                cpu: "0.5"
                memory: 1Gi
        kind: hosted
        name: no-model-agent
        protocols:
            - protocol: responses
              version: 1.0.0
infra:
    provider: microsoft.foundry